Posi temp issue with a curveball

Hey guys, I need some help here. I have a shower with a Moen Posi Temp valve. Its installed in a bathtub/shower combo. Lets say for this example I have the handle directly pointed up...right in between hot and cold. If I let the water run out of the faucet, its a nice warm temp with no issues. I can adjust it colder or hotter and the water temp responds as it should. If I pull the diverter and make the water run out the shower head, it gets full hot without moving the handle at all. If I move the handle to colder, there is no response. There also seems to be a change in the sound of the water rushing through the valve. If I push the diverter back down and let it come out of the faucet again, without touching the handle it goes back to being a nice warm temp like it should. When it goes back to warm, I can hear a change in the tone of the water going through the valve again. I figured it was just a faulty cartridge, so I purchased a brand new cartridge and popped it in. No change.

I suspect the balancer is unable to fully react to some pressure changes due to the large difference in volume flow between shower and tub. It should, of course, so the first step I would do is is get an inexpensive pressure gauge at the hardware store, connect it to a bibb or faucet on the house side of your regulator, and measuer both static ( no water running) and dynamic ( some faucet full on) pressures. These readings may be informative.

Hmm...ok...but what would make this change so suddenly? I can't say for sure, but it seemed to start around the time we put a new wand style shower head on that shower. Neither my wife or I can say for sure that's when it started. Could that cause this issue? When you mentioned volume difference, could the restriction in flow to that shower head cause this? Are posi temp valves sometime incompatible with certain shower heads? I know everything now days is super low flow...

I should also note this. This morning I was taking my shower and before I was done, I turned the handle to almost full cold. The water stayed full hot. It was tolerable, because I turned my water heater way down so we could at least continue showering until this is fixed. Anyway, I turned it to almost full cold, and the water stayed hot. I pushed the shower diverter down and immediately the water coming out of the faucet was full cold. Its so odd! This has be really baffled!

i would call moen and see what they have to say.. they have great customer service from my experience. an easy test is try a different shower head, as this seems to be causing your problem...maybe if you have your old one to pop back on and see if it works

I'll give them a call today and see what they say. I would try the old one, but we threw it away when we changed to the new one. I knew that was a bad idea! Ha ha. Basically, we replaced a large faced normal shower head with a wand type. I tried removing the restrictor in the wand head to see if that made a difference. No luck there either.